Lesson P13 – Digital Input and Output Programming

🎯 Learning Objectives

After completing this lesson, students will be able to:

✅ Understand Digital Input and Output

✅ Understand HIGH and LOW states

✅ Read push buttons and switches

✅ Control LEDs and relays

✅ Use digitalRead() and digitalWrite()

✅ Build interactive Arduino projects

✅ Understand pull-up and pull-down concepts

1. Introduction

Arduino interacts with the outside world using pins.

These pins can work in two modes:

Input

Arduino receives information.

Examples:

  • Push Button
  • Switch
  • IR Sensor
  • Vibration Sensor

Output

Arduino sends information.

Examples:

  • LED
  • Relay
  • Buzzer
  • Motor Driver

This process is called:

Digital Input and Output (Digital I/O)

Digital I/O is the foundation of almost every Arduino project.

2. What is Digital Signal?

A digital signal has only two states:

State Value
LOW 0
HIGH 1

or

State Voltage
LOW 0V
HIGH 5V

Arduino UNO understands only these two states for digital pins.

3. Real-Life Example

Imagine a room light switch.

Switch OFF

↓

Light OFF

Switch ON

↓

Light ON

Only two conditions exist.

This is a digital system.

4. Digital Output

When Arduino sends signals to a device, it acts as an Output.

Examples:

  • LED ON/OFF
  • Relay ON/OFF
  • Buzzer ON/OFF

5. Configuring Output Pins

Before using an output pin:

pinMode(pin, OUTPUT);

must be used.

Example:

pinMode(13, OUTPUT);

Pin 13 is now configured as an output.

6. digitalWrite()

Used to send HIGH or LOW.

Syntax:

digitalWrite(pin, state);

Turning LED ON

digitalWrite(13, HIGH);

Pin Output:

 
5V
 

LED turns ON.

Turning LED OFF

digitalWrite(13, LOW);

Pin Output:

0V

LED turns OFF.

Complete Example

void setup()
{
pinMode(13, OUTPUT);
}

void loop()
{
digitalWrite(13, HIGH);

delay(1000);

digitalWrite(13, LOW);

delay(1000);
}

Result:

LED blinks continuously.

7. Digital Input

When Arduino receives information from external devices, it acts as an Input.

Examples:

  • Push Button
  • Toggle Switch
  • IR Sensor
  • Limit Switch

Configuring Input Pins

Syntax:

pinMode(pin, INPUT);

Example:

pinMode(2, INPUT);

Pin 2 becomes an input.

8. digitalRead()

Used to read digital signals.

Syntax:

digitalRead(pin);

Returns:

HIGH

or

LOW
 

Example

 
int buttonState;

buttonState = digitalRead(2);
 

Arduino stores button status.

9. Push Button Working Principle

Button Released:

 
LOW
 

Button Pressed:

 
HIGH
 

Arduino continuously checks the state.

Example Program

 
void setup()
{
   pinMode(2, INPUT);

   Serial.begin(9600);
}

void loop()
{
   int buttonState;

   buttonState = digitalRead(2);

   Serial.println(buttonState);

   delay(200);
}
 

Output:

 
0
1
0
1
 

depending on button state.

10. Controlling LED Using Push Button

This is one of the most important beginner projects.

Logic

If button pressed

↓

LED ON

Otherwise

↓

LED OFF

Program

 
void setup()
{
   pinMode(2, INPUT);

   pinMode(13, OUTPUT);
}

void loop()
{
   if(digitalRead(2) == HIGH)
   {
      digitalWrite(13, HIGH);
   }
   else
   {
      digitalWrite(13, LOW);
   }
}
 

11. Pull-Up and Pull-Down Resistors

A common beginner problem:

Button not pressed

↓

Random readings appear.

Why?

Because the input pin is floating.

Floating Input

A floating pin is not connected to HIGH or LOW.

Result:

Random values.

Example:

 
1
0
1
1
0
0
1
 

Solution

Use:

Pull-Up Resistor

or

Pull-Down Resistor

12. Internal Pull-Up Resistor

Arduino UNO contains built-in pull-up resistors.

Very useful.

Syntax

 
pinMode(2, INPUT_PULLUP);
 

Now external resistor is usually unnecessary.

Important Note

With INPUT_PULLUP:

Button State Reading
Released HIGH
Pressed LOW

Logic becomes reversed.

Example

 
if(digitalRead(2)==LOW)
{
   digitalWrite(13,HIGH);
}
 

Button press gives LOW.

13. Relay Control

Relays are common digital output devices.

Arduino sends:

 
HIGH
 

or

 
LOW
 

to control a relay.

Example

 
digitalWrite(8,HIGH);
 

Relay ON.

 
digitalWrite(8,LOW);
 

Relay OFF.

Applications

  • Home Automation
  • Pump Control
  • Smart Appliances

14. IR Obstacle Sensor Example

IR Sensor Output:

Obstacle Present:

 
LOW
 

No Obstacle:

 
HIGH
 

(depends on sensor model)

Program Logic

 
if(digitalRead(2)==LOW)
{
   Serial.println("Obstacle Detected");
}
 

15. Vibration Sensor Example

 
if(digitalRead(3)==HIGH)
{
   Serial.println("Vibration Detected");
}
 

Applications:

  • Security Systems
  • Earthquake Monitoring
  • Machine Monitoring

16. Real Project Examples

Smart Door Alarm

Input:

Button Switch

Output:

Buzzer

Automatic Water Dispenser

Input:

IR Sensor

Output:

Relay + Pump

Home Security System

Input:

Vibration Sensor

Output:

Alarm

Smart Dustbin

Input:

IR Sensor

Output:

Servo Motor

17. Common Beginner Mistakes

Mistake 1

Forgetting pinMode()

Wrong:

 
digitalWrite(13,HIGH);
 

without:

 
pinMode(13,OUTPUT);
 

Mistake 2

Using wrong pin numbers.

Mistake 3

Ignoring pull-up requirements.

Mistake 4

Confusing HIGH and LOW logic.

Especially with INPUT_PULLUP.

18. Best Practices

✅ Define pin modes in setup()

✅ Use INPUT_PULLUP when possible

✅ Label pin connections

✅ Test inputs using Serial Monitor

✅ Use comments for clarity

📊 Summary

In this lesson, we learned:

✅ Digital Signals

✅ Digital Input

✅ Digital Output

✅ pinMode()

✅ digitalRead()

✅ digitalWrite()

✅ Push Button Control

✅ Pull-Up Resistors

✅ Relay Control

Digital I/O is the most fundamental concept in Arduino programming because almost every sensor and actuator communicates through digital signals.

📖 Key Terms

Digital Input

Receiving HIGH or LOW signals.

Digital Output

Sending HIGH or LOW signals.

HIGH

Logic 1 (5V).

LOW

Logic 0 (0V).

digitalRead()

Reads digital signals.

digitalWrite()

Writes digital signals.

INPUT_PULLUP

Enables internal pull-up resistor.

Floating Input

Input pin without a defined state.

🎯 Quiz

1. Which function reads a digital pin?

A. digitalWrite()

B. digitalRead() ✅

C. analogRead()

D. pinMode()

2. What voltage represents HIGH on Arduino UNO?

A. 0V

B. 3.3V

C. 5V ✅

D. 12V

3. Which function sends a digital signal?

A. digitalWrite() ✅

B. digitalRead()

C. analogRead()

D. Serial.print()

4. What does INPUT_PULLUP do?

A. Increases voltage

B. Enables internal pull-up resistor ✅

C. Starts communication

D. Controls PWM

5. Which device is a digital input?

A. LED

B. Relay

C. Push Button ✅

D. Buzzer

🏠 Assignment

Task 1

Create a program that turns an LED ON when a push button is pressed.

Task 2

Display button status in Serial Monitor.

Task 3

Control a relay using Arduino.

Task 4

Use an IR sensor to detect an object and display a message.

Task 5

Explain the difference between INPUT, OUTPUT, and INPUT_PULLUP with examples.

➡️ Next Lesson: P14 – Analog Input and PWM Programming (Detailed Practical Lesson), which connects directly to sensors like LDR, potentiometer, MQ gas sensor, and motor speed/LED brightness control.

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